Conventionally, in the lithographic process to manufacture a semiconductor device (integrated circuit), a liquid crystal display device, and the like, various exposure apparatus were used. In recent years, as these types of exposure apparatus, the reduction projection exposure apparatus such as the so-called stepper or the so-called scanning stepper is mainstream, from the viewpoint of having high throughput. With the reduction projection exposure apparatus, a fine circuit pattern formed on a photomask or a reticle is reduced, projected, and transferred onto a substrate such as a wafer or a glass plate, which surface is coated with a photoresist via a projection optical system.
However, the exposure apparatus such as the projection exposure apparatus require high resolution, along with high throughput. The resolution R, and the depth of focus DOF of the projection exposure apparatus are respectively expressed in the following equation (1) and (2), using the wavelength of the illumination light for exposure λ and the numerical aperture of the projection optical system N.A.:R=K·λ/N.A.  (1)DOF=λ/(N.A.)2/2  (2)
As is obvious from equation (1), three ways can be considered to obtain a smaller resolution R, that is, to decrease the minimum pattern line width that can be resolved; {circle around (1)} reduce the proportional constant K, {circle around (2)} increase the N.A., {circle around (3)} reduce the wavelength of the illumination light for exposure λ. The proportional constant K, in this case, is a constant that is determined by the projection optical system or the process, and is normally a value around 0.5 to 0.8. The method of decreasing the constant K is called super-resolution in a broad sense. Up until now, issues such as improvement of the projection optical system, modified illumination, phase shift reticle have been studied and proposed, however, there were drawbacks such as the patterns suitable for application being restricted.
On the other hand, as can be seen from equation (1), the resolution R can be reduced by increasing the numerical aperture N.A., however, at the same time, this means that the depth of focus DOF is small, as is obvious from equation (2). Therefore, increasing the N.A. value has its limits, and normally, the appropriate value is around 0.5 to 0.6.
Accordingly, the most simple and effective way of reducing the resolution R is to reduce the wavelength of the illumination light for exposure λ.
For such reasons, conventionally, the g-line stepper and the i-line stepper that use an ultra-high pressure mercury lamp as the light source for exposure to emit the emission line (such as the g line or the i line) in the ultraviolet light region were mainly used. However, in recent years, the KrF excimer laser stepper that uses a KrF excimer laser as the light source to emit a KrF excimer laser beam having a shorter wavelength (wavelength: 248 nm) is becoming mainstream. And currently, the exposure apparatus that uses the ArF excimer laser (wavelength: 193 nm) as the light source having a shorter wavelength is under development.
The excimer laser, however, has disadvantages as the light source for the exposure apparatus, such as, the size being large, the energy per pulse being large causing the optical components to damage easily, and the maintenance of the laser being complicated and expensive because of using poisonous fluorine gas.
Therefore, the method of utilizing the nonlinear optics effect of the nonlinear optical crystal to convert light with a long wavelength (infrared light and visible light) to an ultraviolet light with a shorter wavelength and using the ultraviolet light as the exposure light, is gathering attention. As the light source employing this method, the array laser which details are disclosed in, for example, Japanese Patent Laid Open (Unexamined) No. 08-334803, is well known. With the array laser, the wavelength of light from the laser beam generating portion comprising a semiconductor laser is converted by the nonlinear optical crystal arranged at the wavelength conversion portion, and a laser element which generates ultraviolet light is bundled into an ultraviolet light source of a plurality of lines in a matrix shape (for example, 10×10).
With the array laser, by bundling a plurality of lines of laser elements that are individually independent, the light emission of the individual laser elements can be suppressed at a low level, while maintaining the light emission of the whole apparatus high. However, since the individual laser elements were independent, fine adjustment was required in addition to an extremely complicated structure in order to make the oscillation spectrum of each laser element coincide with one another.
And so, the method to convert the wavelength can be considered where the laser beam emitted from a single laser oscillation source is diverged, and the wavelength of each diverged beam is converted with a common nonlinear optical crystal after each diverged beam is amplified. In the case of employing this method, it is convenient to use optical fiber to guide the laser beam, and the arrangement of a plurality of bundled optical fibers emitting a plurality of beams incident on the nonlinear optical crystal is the most suitable from the viewpoint of simple arrangement, smaller diameter of the emitting beam, and maintenance operation.
In addition, to efficiently generate a second harmonic and the like by the nonlinear optics effect using the nonlinear optical crystal, a linearly polarized beam of a specific direction which corresponds to the crystal direction of the nonlinear optical crystal needs to be incident on the nonlinear optical crystal. However, it is generally difficult to arrange the direction of the linearly polarized beams emitted from a plurality of optical fibers in order. This is because even if the polarization maintaining fiber is used to guide the linearly polarization, since the sectional shape of the optical fiber is almost round, the direction of the linearly polarization cannot be specified from the outside shape of the optical fiber.
Also, as is well known, in the case of using an excimer laser beam in the short wavelength region, mainly due to the transmittance of the material, the material that can be used for the lens of the projection optical system at this stage is limited to materials such as synthetic quartz, fluorite, or fluoride crystal such as lithium fluoride.
In the case of using lenses made of materials such as quartz or fluorite in the projection optical system, however, correction of chromatic aberration is actually difficult. Therefore, in order to prevent the image forming performance from deteriorating, narrowing the oscillation spectral width of the excimer laser beam, in other words, to narrow-band the wavelength is required. To perform this narrow-banding, for example, a narrow-band module (optical elements such as a combination of a prism and a grating (diffraction grating) or an etalon) arranged in a laser resonator is used, and it becomes necessary to keep the spectrum width of the wavelength of the excimer laser beam supplied to the projection optical system during exposure within a predetermined wavelength width at all times, and at the same time, the so-called wavelength stabilizing control to maintain the center wavelength at a predetermined wavelength becomes required.
In order to achieve the wavelength stabilizing control referred to above, the optical properties of the excimer laser beam (such as the center wavelength and the spectral half-width) need to be monitored. The wavelength monitor portion of the excimer laser unit is made up of a Fabry-Perot etalon (hereinafter also referred to as an “etalon element”) playing the main role, which is in general a Fabry-Perot spectroscope.
In addition, with higher integration of the semiconductor device, the pattern line width is becoming finer, and further improvement on exposure accuracy such as the overlay accuracy of the mask and the substrate in the exposure apparatus such as the stepper is being required. The overlay accuracy depends on how well the aberration of distortion components and the like in the projection optical system is suppressed. Therefore, the center wavelength stability of the illumination light for exposure and further narrow-banding is becoming required in the exposure apparatus. Of these requirements, as a method of coping with narrow-banding, employing a single-wavelength light source as the laser light source itself can be considered.
Meanwhile, since the projection optical system is adjusted only to a predetermined exposure wavelength, if the center wavelength cannot be stably maintained, as a consequence, chromatic aberration of the projection optical system may occur, or the magnification of the projection optical system or the image forming characteristics such as distortion and focus may vary. Therefore, it is a mandatory to maintain the stability of the center wavelength.
However, since the etalon element is affected by the temperature and pressure of the etalon atmosphere, the influence of the change in temperature and atmospheric pressure in the etalon atmosphere cannot be ignored.
In addition, it is certain that a finer device rule (the practical minimum line width) will be required in the future, and the exposure apparatus of the next generation will require higher overlay accuracy than before. The overly accuracy depends, for example, on how well the distortion component is suppressed. Also, in order to increase the depth of focus, increase in the UDOF (usable DOF) and stability in focus will be necessary. And in both cases, stability of the center wavelength and controllability of the spectral half-width are required at a high degree.
Also, the exposure apparatus will be expected to achieve exposure amount control performance in line with the difference of the resist sensibility in each wafer, and a wide dynamic range, typically around 1 to 1/7, will be required. With the exposure apparatus using the conventional excimer laser as the light source, for example, the rough energy adjuster such as the ND filter is used for exposure amount control in accordance with the difference of the resist sensibility in each wafer.
In the case of such a method, however, an ND filter with a calibrated transmittance was required, and the durability of the ND filter and the change in transmittance with the elapse of time caused a problem. Furthermore, even in the case when only 1/7 of the maximum exposure light amount was required, the excimer laser operated to emit the exposure light at the maximum intensity, therefore, 6/7 of the emitted light was not used upon exposure, and was wasted. And, there were also difficulties on points such as the optical components wearing out and power consumption.
With the current exposure apparatus, other than the exposure amount control performance in accordance with the difference of the resist sensibility in each wafer (hereinafter referred to as the “first exposure amount control performance” as appropriate), the exposure amount control performance to correct the process variation of each shot area (chip) on the same wafer (hereinafter referred to as the “second exposure amount control performance” as appropriate) is required. Also, in the case of the scanning stepper, the exposure amount control performance to achieve line width uniformity within the shot area (hereinafter referred to as the “third exposure amount control performance” as appropriate) is further required.
With the current exposure apparatus, as the second exposure amount control performance referred to above, the dynamic range is required to be around ±10% of the exposure amount set, the exposure amount is required to be controlled within about 100 ms, which is the stepping time in between shots, to a value set, and the control accuracy is required to be around ±1% of the exposure amount set.
And, as the third exposure amount control performance referred to above, the control accuracy is required to be set at ±0.2% of the exposure amount set within 20 ms, which is the typical exposure time on one shot area, with the control velocity around 1 ms.
Accordingly, as the light source of the exposure apparatus, in order to achieve the first to third exposure amount control performance described above, the advent of a light source unit that can perform control corresponding to necessary requirements for control is highly expected. Control corresponding to necessary requirements for control, here, refers to functions such as (a) dynamic range of control, (b) control accuracy, (c) control velocity, (d) degree of linearity between the detected light intensity and the control amount, and (e) energy saving functions for the purpose of power-saving.
The present invention has been made in consideration of the situation described above, and has as its first object to provide a light source unit that can perform light amount control corresponding to necessary requirements for control described above.
It is the second object of the present invention to provide a light source unit that can maintain the center wavelength of the laser beam at a predetermined set wavelength without fail.
It is the third object of the present invention to provide a light source unit with a simple arrangement that can generate a predetermined light while controlling the polarized state.
It is the fourth object of the present invention to provide a wavelength stabilizing control method that can maintain the center wavelength of the laser beam at a predetermined set wavelength without fail.
It is the fifth object of the present invention to provide an exposure apparatus that can easily achieve the exposure amount control required.
It is the sixth object of the present invention to provide an exposure apparatus that can perform exposure with high precision without being affected by the temperature change and the like in the atmosphere.
It is the seventh object of the present invention to provide an exposure apparatus that can perform exposure with sufficient accuracy regardless of the change in sensitivity properties of the photosensitive agent.
It is the eighth object of the present invention to provide an exposure apparatus that can efficiently transfer a predetermined pattern onto a substrate.
It is the ninth object of the present invention to provide an exposure method that can easily achieve the exposure amount control required.
It is the tenth object of the present invention to provide an exposure method that can perform exposure with high precision without being affected by the temperature change and the like in the atmosphere.
And, it is the eleventh object of the present invention to provide a device manufacturing method that can improve the productivity of the micro device with high integration.